Yesterday evening, we had again perfect Ultrasonic conditions: 8 (average) - 14 (peak) knots. Typical evening breeze.
I used my 10.9 sail.
Early this year, I bought an older Deboichet R12 / 64 fin which I used now for the first time.

I can recommend the fin to everyone who likes to sail the US147 in very light conditions.
It is a very soft fin with lots of lift in light wind. Free of spin-out and it points very high. Much better than the standard R13/58 fin.

I can see that sails are getting bigger when using boards lika Ultrasonic. JP was a little bit earlier with JP SLW and I bought it some years ago. I do not want to big Formula sails and I have put my limit just above 9. With TR 4, 9.2 and Gaastra Vapor 7.6 I can manage most winds the board are made for and I don not need a bigger sail. Increasing winds and to big sails are not fun! You are planning anyway in 5 m/s with 9.2 so why get a big Formula-sail when not sailing a Formulaboard?

With my 87 kg, I probably plane even with my 8.8 in 5 m/s = +/- 10 knots. For sure in a downwind slalom course.

The thing is that I also want to plane upwind in 8 knots average. That also means regular lulls of 6 knots or less.
Therefore, I prefer a 10.9 sail. It's no fun to be off and on the plane, nor to have problems to return to the starting point.
On the other side, I don't like too much freeride sails that quickly overpower me when the wind picks up. I had before a 9.5 freeride/freerace sail which horrified me in stronger winds. I'm far more comfortable with my 10.9 Formula sail than with my former 9.5 sail, despite of the heavier weight.

Good question. I don't know. I've never sailed an e.g. IS127/137. So I can't compare.

Perhaps Remi or Jean-Marc can answer this one.

My guess is that on reaching courses and typical PWA downwind slalom courses, there will be not much in it. Probably the US needs less pumping. On the other hand, in slightly upwind courses it could be a different story.

If I compare the US and Formula in upwind conditions, I find there is still quite a difference. Formula rules without any doubt.

Dear MOP, yes I do have the same question too .. more even since my weight is 72-75 kg [ no clothes] with a wetsuit ..we are a lot heavier of course not to mention boom/mast/sail.
Is a ULTRASonic no overkill for me .. r would it just work .. is an ultrasonic not only for heavier riders .. do you need as many liters for 75 kg surfer to plane as quick as possible or is the width that makes the biggest difference?
WOuld an IS 127 not better for me. I currently sail a FAnatic ray 145 ltd [2011] with a 9m Maui Sails Titan and it gets big quickly. I just want to plane earlier and have more control in the gusts.
I know formula is the king .. but I wanna have lot's of fun to and a board that serves me well for years to come.